(comp): GameBoySubtargets

[arkamnite]

Implement, or exclude, the original Game Boy as a subtarget.

This should arguably take place as late as possible, as subtargeting is not a requirement of this project at this stage. Additionally, it may transpire that there is little difference that takes place within the backend when it comes to distinguishing between these two devices; most differences will take place in the SDK.

The task is therefore to:

• Identify how much existing subtarget code can either be removed entirely, or modified to support a GameBoy DMG unit.